Modeling the effects of soil structure dynamics on soil hydraulic properties.

Soil structure is shaped by the dynamic interaction of physical, chemical, and biological processes, functioning across a wide range of spatial and temporal scales. Alterations in soil structure can, in turn, regulate essential soil processes such as the transport and reactivity of solutes, evaporation and water fluxes, microbial activity, etc., ultimately influencing biogeochemistry in soils and ecosystem functioning. However, the majority of modern widely used predictive models do not account for these dynamics and treat soil structure and thus soil hydraulic properties (SHP) as static. Recently, Jarvis et al. introduced the USSF model, a framework designed to integrate dynamic changes in soil structure and the consequent evolution of SHP. In this contribution, we build on the hydrological component of the USSF model to enhance its flexibility and enable a more accurate representation of SHP in both the soil matrix and structural part.

The USSF model simulates soil matrix water flow using the Brooks–Corey formulation, which assumes a non-zero residual water content at oven-dry matric potentials and exhibits physically inconsistent variability in the extremely dry region of the water retention curve. The soil structural domain is represented through an empirical macropore model with fixed boundaries of the structural pore size distribution. We introduce a more flexible and physically consistent description of matrix SHP based on the Brunswick model, coupled with a fracture-domain hydraulic formulation derived from the Tuller-Or model to represent structural pore flow.

The extended model was evaluated for two contrasting agricultural management strategies: direct seeding (DS) and conventional tillage (CT), with both systems initialized using a 10-year conventional tillage warm-up period. For both systems, soil organic matter was the primary driver of long-term porosity dynamics, with the direct seeding system reaching equilibrium within 10 years in the former plough layer, at 0-25 cm depth. The simulated SHP profiles aligned with published data, capturing a transient post-tillage increase in saturated hydraulic conductivity (Ksat) under CT, followed by rapid structural settling. Under DS temporal Ksat variability was lower. The Ksat depth profile within the plough layer remained vertically uniform in CT, whereas DS showed a systematic decline of Ksat with depth. The model enables realistic reconstruction of how agricultural operations will affect structural porosity and SHP. Future development will couple the extended model with a broader soil-crop-atmosphere system model and focus on improving the process description, particularly regarding seasonal dynamics.